Re: [CAD_CAM_EDM_DRO] First note-Help Mariss Help us with shielding???

Christopher Prosser wrote:

> Hi Tom,
>
> > Electrical noise has been a real pain for me in many projects I have
> worked
> > on.
> >
>
> Thanks for all your advice so far. You didn't mention where you grounded the
> encoder (signal level) cable. DC Brush motors are super dirty.

Actually, the motors are NOT all that dirty. Well made (brush) servo motors
do not exhibit a bunch of sparking at the commutator like vacuum cleaner
and small tool motors. The 25 KHz, up to 80 V or so PWM waves with rise
times below 100 nS ARE very dirty, and for cost reasons, the Gecko drives
can't include output filters.

> For all the
> robotics stuff I've done, I've always made the motor power supply completely
> separate from the logic power supply. I would run the voltage regulator for
> the electronics off a completely separate battery.

Battery? You are going to have battery powered units in a CNC machine tool?
Will you have an early warning that puts the machine into E-stop before the
logic fails, as the battery drains?

> Has anyone placed a cap across the servo motor terminals? In all my work
> with much smaller DC motors, I've always placed a cap across the terminals
> at the motor to help soak up noise the motor wants to inject back into the
> power supply. But, I've done PWM at 1KHz to control the speed of these
> motors, not the 25KHz that the Gecko's use. I don't know if there would be a
> problem with heating the cap or causing something bad to happen with the
> amount of power involved here.

Placing a good sized capacitor across the motor terminals of a PWM full
bridge switch is a recipe for disaster! It will cause current surges possibly
up to hundreds of amps 50,000 times a second. If the capacitor doesn't
explode, the power transistors definitely will! The only way to do this is
to include inductors between the power transistors and the capacitor.

> The fact that grounding the motor power cable helped, implies that some
> nasty spikes are coming out of the motor and confusing everything around it,
> not necessarily just the encoder logic. Just my interpretation of the runes.

No, not the MOTOR, but the drive itself. It is, in fact, provable that the
unfiltered PWM output MUST be worse than any noise the motor could
possibly create!

>
> >From www.control.com
> "Usually it is a contact opening an inductive load such as a motor. You can
> purchase suppressors to put right across the motor. The suppressor is
> usually a resistor/capacitor series combination. IEEE put out an electrical
> noise guide book which discusses the many forms of noise and how to cope
> with them.
> "

Even here, they don't recommend a capacitor right across the motor, but
with a resistor in series with the capacitor.

Jon